The melt-reduction is a method of directly producing molten iron or molten pig iron by throwing iron bearing material, carbon material and flux into a furnace body, blowing pure oxygen and/or an oxygen-rich gas thereinto, and reducing iron oxides of the iron bearing material in the slag. According to this method, combustible gases at temperatures as high as about 1600 to 1800.degree. C. are produced from the melt-reducing furnace.
In general, the method of melt-reduction of this kind can be divided into a two-stage method according to which pre-reduced iron bearing material, carbon material and flux are thrown into the furnace body, and the iron ore is pre-reduced with a CO gas and an H.sub.2 gas contained in the combustible gases generated from the furnace body, and a single-stage method according to which unreduced iron bearing material, carbon material and flux are thrown into the furnace body, iron oxides in the iron bearing material are reduced in the slag, a CO gas and an H.sub.2 gas in the combustible gases generated from the furnace body are completely burned in a waste heat boiler, and the sensible heat and the latent heat of the combustible gases are recovered by vaporization to generate. electricity (see, for example, Japanese Unexamined Patent Publications (Kokai) No. 1-502276, No. 63-65011, No. 63-65007, etc.).
The two-stage method has an advantage of better energy efficiency than the single-stage method, but requires a pre-reducing furnace such as of a packed bed type or a fluidized bed type, causing the facility to become complex, requiring an increased investment for the facility, and imposing limitation on the shape of iron bearing material due to uniform reaction in the pre-reducing furnace (e.g., the packed bed system permits the use of massive iron bearing material only, and the fluidized bed system permits the use of powdery iron bearing material only). In recent years, therefore, a simple single-stage method has drawn attention.
It has been widely known that in the single-stage method, the energy efficiency is improved, i.e., the unit requirement of carbon material is decreased by increasing the rate of combustion of CO gas and H.sub.2 gas generated in the slag (hereinafter referred to as secondary combustion rate in the furnace, which is defined to be (CO.sub.2 %+H.sub.2 O%)/(CO.sub.2 %+CO%+H.sub.2 O%+H.sub.2 %)) in a space in the furnace over the slag to effectively transmit the heat of combustion to the slag, and that the amount of heat of the combustible gases, i.e., the sum of the sensible heat and the latent heat generated from the furnace body, decreases by an amount by which the unit requirement of carbon material is decreased.
In the single-stage method as shown in FIG. 4, what is important is to compensate for the energy efficiency which is inferior to that of the two-stage method by recovering by vaporization the sensible heat and the latent heat of combustible gases generated in large amounts from the furnace body to generate electric power which can be sold to the utility or which can be used in other facilities in the factory, contributing to decreasing the amount of electric power that must be purchased.
In order to repair furnace body refractories of the melt-reducing furnace, however, the operation must be halted at regular intervals, e.g., once in three to twelve months as shown in FIG. 5. That is, no electricity is generated during the period in which the operation is halted leaving a problem from the standpoint of stably supplying electric power. For example, when the electric power is to be sold to the utility, the price must be set low or when the electric power is to be used in other facilities in the factory, operation of the other facilities in the factory is interrupted.
The present invention was accomplished in order to solve the above-mentioned problems, and its object is to stably supply the electric power even when the operation is regularly halted in order to repair furnace body refractories in the melt-reducing furnace.